A number of years ago a fellow I knew had a Marlin, I am pretty sure it was a Model 120 with a 40 inch barrel.Just wondering what the reason was that Marlin made a shotgun with so long of a barrel?Also does anyone have any idea of how many of these 40 inch Model 120 Marlins were manufactured?

The Marlin Model 120 MXR Magnum shotgun was made from 1974 to 1975. There were a total of 6,984 of the 40 inch barreled guns made at that time. It sold for $158.95 in 1974.
The 40 inch barrel was advertised by Marlin to "reach out for the long ones, and shoots quieter and balances surprisingly well." The reaching out further is not true, but it does sound good on paper.
I hope this helps.

Due to the increasing cost of ammunition, there be no warning shot!

The growing federal deficit = generational slavery to the national debt.

The Marlin Model 120 was made from 1971 to 1985. It was made in several styles:
The 120 Trap Gun of which only 298 were made.
The 120 MXR Magnum with 40 or 38 inch barrels...6,984 made
The 120 Slug gun.....1,546 made
The 120 Field Grade.....1,083 made
The 120 Deluxe Grade........ 10,689 made
and if you want to include it, there is the Glenfield Model 778 which is the cheaper version of the Modle 120, with 34,941 produced from 1979 to 1983. Marlin made 1,238 of the 778 with the longer barrels. There was a total of 48,651 of the Model 120 shotgun manufactured during the production run.
Barrel lengths available: 20 or 26 inch slug barrel, 26 inch, 28 inch, 30 inch and the 40 & 38 inches.

As with any Marlin product, it is the bottom line that probably spelled the end of the Model 120. If it did not produce enough profit, it would be dropped from the line.
I hope this helps.

Due to the increasing cost of ammunition, there be no warning shot!

The growing federal deficit = generational slavery to the national debt.

I have just picked up a Marlin 120 with a 40" barrel and was wondering what the DOM might be #A62031. What grade would this shotgun be? I didn't realize the 40" barrels were uncommon, 6984 produced . The gun also came with 2 extra barrels and the gun shows very little use.

Hi Road King,
Based on your serial number, it is likely manufactured in late 1977 or 1978. The Field Grade models were plain barrel (lacking rib) in 28" modified, produced in 1978 only. The MXR model with 40" barrel was manufactured 1974-75, so I suspect your barrel was purchased later from GunPartsCorp, who acquired a large number of original Marlin barrels for this model after discontinuation. The original configuration for your shotgun could be a field grade OR a deluxe grade model, depending on which barrel was original matched and sold with the receiver! (note the Deluxe grade was offered 1978-82, having hand cut checkering the last 2 years only)

The plain 28" barrel shown on the top is the barrel that I believe the gun came with, maybe the ribed barrel and yes the 40" barrel was purchased seperatly. The original owner has passed away and not sure if any of the family knows which barrel the gun came with. I am away for a few days but will examie the barrels when I get get home to see if I can determine the original barrel the gun may have came with.